Nickel and cobalt are generally found together in natural-occurring minerals and, because conventional ore dressing methods do not effect a separation of the two, both metals generally appear together in solutions resulting from the leaching of nickel and cobalt-containing materials, such as leached oxide ores, sulfide concentrates and the like.
In recent years, several hydrometallurgical methods have been proposed for the recovery of nickel and/or cobalt from lateritic limonitic ores. One method, in particular, resides in using aqueous sulfuric acid as the leachant at high temperature under elevated pressure, the raw ore being prepared in a finely divided state, then forming a slurry thereof at about 10 percent to 20 percent solids which is thereafter concentrated by settling and decanting in thickeners to produce an underflow having a concentration of about 30 percent to 50 percent solids. The concentrated slurry is heated in an autoclave by means of direct high pressure steam to a high temperature at which the leaching or other recovery treatment is carried out, usually above 400.degree.F (205.degree.C), e.g. about 475.degree.F (246.degree.C), at a pressure of about 525 psig in the presence of sulfuric acid to solubilize the nickel and cobalt present in the slurried ore. Following leaching in the autoclave, the leached pulp is cooled and preferably washed by countercurrent decantation and the resulting acid leach liquor then treated with a neutralizing agent [Mg(OH).sub.2, coral mud or the like] to raise the pH to, for example, 2.5 to 2.8 for the sulfide precipitation of nickel and cobalt. The leach liquor is brought to a temperature of about 250.degree.F (122.degree.C) and the nickel and cobalt precipitated as sulfides with H.sub.2 S at pressures of up to about 150 psig, using nickel sulfide as seed material.
The sulfide precipitate is washed and thickened to about 65 percent solids and then oxidized at about 350.degree.F (177.degree.C) and a pressure of about 700 psig in an autoclave in 1 percent sulfuric acid. Ammonia is added as a neutralizing agent to the nickel-cobalt solution to raise the pH to a level (e.g. 5.3), using air as an oxidant, to precipitate any iron, aluminum or chromium carried over as an impurity during leaching. After separating the solution from the precipitate, any copper, lead or zinc present therein is removed by precipitation as a sulfide, using H.sub.2 S as the precipitant, the solution being first adjusted with acid to lower the pH to about 1.5. The sulfide precipitate is then separated from the solution and the solution adjusted with ammonia to prepare it for the recovery of metallic nickel. The adjusted nickel feed solution containing about 40 to 50 grams per liter of nickel and some cobalt is reduced with hydrogen in an autoclave at about 375.degree.F (190.degree.C) and 650 psig using nickel powder as seed material, the barren liquor remaining going to cobalt recovery using known methods. However, some of the cobalt appears in the reduced nickel product.
Among the methods proposed and/or commercially used for separating cobalt from nickel is the method of separating cobalt from aqueous nickel-cobalt sulfate solutions by means of nickelic hydroxide, and the subsequent separation of cobalt from nickel in the resulting precipitate by means of the pentammine process described in U.S. Pat. Nos. 2,767,053 and 2,767,054, the cobalt in the precipitate being in the cobaltic state.
The cobaltic precipitate which also contains nickel is converted to cobaltic pentammine and nickelous ammine by adding an amount of ammonia sufficient to provide 5 mols NH.sub.3 for each mol of cobaltic ion and 5 mols of NH.sub.3 for each mol of nickelous ion. The solution is oxidized with oxygen and then acidified with sulfuric acid to a pH of at least 4 to produce a nickel-bearing precipitate in the form of a nickel-ammonium double salt, the double salt being formed in a crystallizer.
While the foregoing technique is useful in the extraction and recovery of cobalt from mainstream nickel-cobalt sulfate solutions, it has certain disadvantages. For one thing, the cost of reagents for forming the pentammine is high. For another, the solutions generally require oxygenation in order to inhibit the reducing effect of sulfuric acid on cobaltic ions, since cobaltic ions are essential in carrying out the pentammine process.
We have discovered an improved method for treating cobaltic precipitates, wherein dissolution of the cobaltic precipitate is effected quickly and completely by utilizing residual liquor from the ammonium sulfate crystallizer.